Mercury vapor discharge lamp and pressure-regulating means therefor

ABSTRACT

The mercury vapor pressure within an operating fluorescent lamp or similar device is controlled by a quantity of mercuryamalgamative material that is retained at a predetermined location within the lamp by holding means secured to one of the mount assemblies or the lamp envelope. The amalgamative material preferably comprises indium and is held in place by a formaminous assembly. In the case of indium, the formed indium-mercury amalgam contains from about 80 to 95 atomic percent indium and both the lamp performance (light output versus ambient temperature) and amalgam retention are significantly improved with indium contents at the high end of the aforesaid atomic percent range. Various amalgam-holding structures, including temperature-compensating types employing bimetal elements, and a method of fabricating the vapor-pressure control components are also disclosed.

United States Patent [72] inventor George S. Evans Caldwell, NJ. [2]] Appl. No. 381,503 [22] Filed July 9, 1964 [45] Patented Nov. 9, 1971 [73] Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

[ 54] MERCURY VAPOR DISCHARGE LAMP AND PRESSURE-REGULATING MEANS THEREFOR 20 Claims, 27 Drawing Figs. [52] U.S. 313/174, 313/109, 313/178 [51] Int. Cl 1101] 61/24 [50] Field of Search 313/174, 175,176,l77,l78,179,180,181,182, 223, 224, 227, 228, 229, 1 l3, 1 14, 109; 315/108 [56] References Cited UNITED STATES PATENTS 3,392,298 7/1968 Menelly 313/109 3,263,111 7/1966 Doering 313/109 Primary Examiner-Raymond F. l-lossfeld Attorneys-A. T. Stratton, W. D. Palmer and D. S. Buleza ABSTRACT: The mercury vapor pressure within an operating fluorescent lamp or similar device is controlled by a quantity of mercury-amalgamative material that is retained at a predetermined location within the lamp by holding means secured to one of the mount assemblies or the lamp envelope. The amalgamative material preferably comprises indium and is held in place by a formaminous assembly. in the case of indium, the formed indium-mercury amalgam contains from about 80 to 95 atomic percent indium and both the lamp performance (light output versus ambient temperature) and amalgam retention are significantly improved with indium contents at the high end of the aforesaid atomic percent range. Various amalgam-holding structures, including temperaturecompensating types employing bimetal elements, and a method of fabricating the vapor-pressure control components are also disclosed.

PATENTEUNHV 9 3, 6 1 9 .69 7

sum 1 [IF 5 INDIUM -44 INVENTOR George S. Evans y ib 'w PATENIEUNOV 9 WI 3.6 1 9 697 sum 2 or 5 FIG.9.

PATENTEBunv 9 I9" PERCENT BR IGHTNESS PERCENT BRIGHTNESS IOO-L 3O 5O 7O 90 HO I30 I50 AMBIENT TEMPERATURE IN F AMBIENT TEMP. RANGE IOO- A 92.5 ATOMIC I NQLQM 95 7 a s7 ATOMIC IN y 83 80% A QMJQEBIHM 60 4O 20 N 20 4O 60 RELATIVE AMBIENT TEMP. IN "F SHEET 4 BF 5 FIG. 24.

CONVENTIONAL LAM P 82 (END CHAMBER) In-Hg AMALGAM LAMP FIG.25.

MERCURY VAPOR DISCHARGE LAMP AND PRESSURE- REGULA'I'ING MEANS THEREFOR This invention relates to mercury vapor discharge lamps and has particular reference to an improved fluorescent lamp having a mercury vapor pressure regulating means that permits the lamp to be operated efficiently at high power loadings or under a wide range of ambient temperatures, or both.

As is well known, the efficiency of a fluorescent lamp is at a maximum when the mercury vapor pressure within the lamp is maintained at approximately 6 to l microns. At this vapor pressure, the amount of 2,537 A. radiation produced by the discharge is at maximum. In fluorescent lamps of conventional loading the design parameters are so correlated that the required mercury vapor pressure prevails under normal operating conditions. However, when the lamp is operated at higher power loadings or under high ambient temperature conditions, the mercury vapor pressure increases and the light output drops off sharply.

In one type of highly loaded fluorescent lamp now being marketed, this basic problem is partially solved by mounting a heat deflecting shield on one (or both) of the lamp stems to provide a cool region within the lamp that maintains the mercury vapor pressure at the proper value. Highly loaded lamps utilizing a shielded or relatively cool region as the mercury vapor control center, however, provide only a partial answer to the problem since their light output is dependent upon and greatly influenced by ambient temperature conditions. Thus, when a conventional highly loaded fluorescent lamp is operated in a semienclosed or totally enclosed fixture, the mercury vapor pressure increases to such a value that a marked decrease in the light output and efficiency results. In addition, since the temperature of the cool region within the lamp is also dependent upon lamp loading, this approach to the problem places a limit on the loading that can be used.

In order to avoid these limitations, it has been proposed that the mercury vapor pressure within a fluorescent lamp be controlled by an amalgam of a selected metal and mercury. Since the vapor pressure of mercury is lower in the case of a mercury amalgam than it is for pure mercury, the use of such an amalgam affords the distinct advantage of controlling the mercury vapor pressure within the lamp without resorting to end chambers or the like. Such an amalgam-containing fluorescent lamp is disclosed in US. Pat. No. 3,007,071 entitled Low- Pressure Mercury Vapor Discharge Lamp," issued Oct. 31, 1961 to A. Lompe et al. While this approach afforded many advantages, it presented rather serious manufacturing problems in that the amalgam had to be coated over a rather large surface area and could not be placed near the electrodes, as indicated in the aforesaid patent. In addition, the amalgam contained relatively large amounts of mercury and thus had a tendency to melt and migrate from the desired location within the lamp during the later stages of lamp manufacture and during operation under extremely high ambient temperature conditions.

It is accordingly the general object of the present invention to avoid the foregoing and other problems associated with the use of an amalgam as a mercury vapor control means in a gaseous discharge lamp.

Another and more specific object is the provision of an improved fluorescent lamp that can be operated at high loadings and over a wide range of ambient temperatures with a minimum loss of light output.

An additional object is the provision of an amalgam-containing fluorescent lamp that can be conveniently manufactured on a mass production basis.

The foregoing objects, and other advantages which will become apparent as the description proceeds, are achieved according to the invention by utilizing a relatively small amount of amalgam and placing it at a selected localized region within the lamp such that it will operate at the temperature required to maintain the desired mercury vapor pressure. The amalgam is held in the proper location within the lamp by a support assembly that is attached to one (or both) of the stems or to the envelope while the lamp is being fabricated.

According to a preferred embodiment, the amalgam-forming metal is sandwiched between two strips of wire cloth. The cloth is made from a metal that the amalgam will wet and the openings in the cloth are such that the amalgam, even when in a liquid state, will be retained within the cloth matrix by capillary action. The metal-cloth sandwich when placed within the lamp does not contain any mercury. The amalgam is formed after the lamp has been dosed with a predetermined amount of mercury in the regular fashion.

Another important feature of the invention resides in maintaining a critical balance between the mercury and parent metal content of the amalgam. The amalgam composition is such that the amalgam will remain substantially in the solid phase when the lamp is not in use and will remain in a semiliquid phase over practically the entire range of temperatures which prevail at the amalgam location when the lamp is operated. In a preferred embodiment, an indium-mercury amalgam is used and the atomic percent indium is maintained within a selected critical range. Specifically, the amalgam contains from about to about atomic percent indium. Surprisingly, this particular amalgam not only has the desired physical properties at the operating and nonoperating temperatures but maintains the light output at higher levels over a wider range of operating temperatures. Lamps containing this particular amalgam will thus operate efliciently over a wide range of ambient temperatures and at loadings significantly higher than those now used.

Various structures and assemblies for holding the amalgam in the desired location within the lamp are provided including one wherein a bimetallic element is utilized to compensate for temperature variations which may be encountered during lamp operation. A preferred method for fabricating a mercury vapor control assembly utilizing strips of metal wire cloth, a strip of amalgam-forming metal, and a pair of rollers is also provided.

A better understanding of the invention will be obtained by referring to the accompanying drawings, wherein:

FIG. I is a front-elevational view of a highly loaded fluorescent lamp embodying the present invention, portions of the bulb being removed for convenience of illustration;

FIG. 2 is an enlarged perspective view of the electrode mount structure of the aforesaid lamp that carries the vaporpressure control assembly of the present invention;

FIG. 3 is a sectional view through the mount along the line Ill-III of FIG. 2;

FIG. 4 is an enlarged plan view of the laminated vapor-pressure control assembly shown in FIGS. 1-3, portions of the assembly being removed to indicate the various layers;

FIG. 5 is a perspective view of the clamp used to hold the aforesaid assembly in position on the stem;

FIG. 6 is a side view illustrating a preferred method for making the laminated cloth-metal assembly shown in FIG. 4;

FIG. 7 is an enlarged fragmentary view of the cloth-metal assembly produced by the method illustrated in FIG. 6;

FIG. 8 is a perspective view of another fluorescent lamp mount embodying an alternative form of vapor control assembly;

FIG. 9 is a plan view of the laminated cloth-metal component of the pressure control assembly shown in FIG. 8, portions of the various layers being omitted for illustrative pur- P FIGS. I0 to 2| are views illustrating alternative holding means and arrangements for mounting the mercury vapor pressure control assembly within the desired location within the lamp;

FIG. 22 is a phase diagram of the mercury-indium alloy system indicating the preferred range of indium content in the amalgam according to the invention;

FIG. 23 is a graph illustrating the diflerence in the operating temperature of the amalgam when the latter is placed on the stem rather than on the bulb wall;

FIG. 24 is a graph comparing the brightness versus ambient temperature characteristics of a conventional highly loaded fluorescent lamp having end chambers and an indium-mercury amalgam lamp embodying the present invention;

FIG. 25 is a graph illustrating the improved brightness versus ambient temperature characteristics displayed by indiummercury amalgam lamps containing increased amounts of indium',

F [G 26 is a graph illustrating the brightness versus ambient temperature characteristics of a family of lamps having the amalgam mounted at different distances from the electrode in accordance with the invention; and

FIG. 27 is a graph contrasting the ambient temperature range versus brightness-maintenance characteristics of the improved indium-mercury amalgam lamps of the present invention and conventional highly loaded lamps in which the vapor pressure is controlled by the temperature of condensed mercury.

EMBODIMENT F FIGS. 1-4

Referring now to the drawings in detail, in FIG. 1 there is shown a highly loaded fluorescent lamp 28 having a tubular light-transmitting envelope 29 which has the usual re entrant mounts 30 sealed into each of its ends. Each of the mounts carries a thermionic electrode 32 that is connected by lead wires 34 and 34' to recessed contacts housed within a suitable base 35 attached to each end of the envelope 29. The inner surface of the envelope is coated with a layer 33 of an ultraviolet-responsive phosphor and one of the mounts 30 is provided with a tubulation 36 that is tipped off in the usual manner after the lamp has been evacuated, mercury-dosed, and filled with a suitable inert starting gas such as argon, neon, or a mixture thereof. In accordance with the preferred em bodiment of the present invention, one of the mounts 30 (preferably the nontubulated mount as shown in FIG. 1) is provided with a mercury vapor pressure control structure such as an arcuate assembly 40 which includes an amalgam-forming metal and is fastened to the mount.

As is illustrated more clearly in H0. 2, the mount 30 comprises the usual flared vitreous stem 31 and lead wires 34 and 34' which support the electrode 32 and are sealed through a press 37 formed on one end of the stem. The electrode 32 preferably consists of a linear triple coiled tungsten filament that carries the usual alkaline-earth oxide coating. A pair of enlarged metal anodes 38 and 38' are also mounted on either side of and in parallel relationship with the electrode 32, preferably by attaching them to the ends of the lead wires.

As shown in FIG. 4, the mercury vapor pressure control as sembly 40 initially comprises a rectangular lamination consisting of a layer 44 of suitable amalgam-forming metal sandwiched between two strips 42, 43 of metal wire mesh or cloth. The strips of wire cloth thus serve as a support structure or matrix for the amalgam-forming metal. As illustrated in FIGS. 2 and 3, in accordance with this form of the invention the aforesaid laminated assembly 40 is wrapped around the cylindrical portion of the stem 31 and tightly clamped therearound by a resilient wire ring 41 (FIG. The assembly is so positioned that the leading edge of the assembly is spaced a predetermined distance d from the transverse plane that passes through the intermediate segments of the lead wires 34, 34' fastened to the ends of the electrode 32. In the case of a linear electrode of the type here shown, the aforesaid plane will of course also pass through the electrode.

The laminated assembly 40 when thus mounted on the stem 3| constitutes a collar that is readily and securely fastened directly to the electrode mount 30. To facilitate this operation, the length of the laminated assembly 40 is made slightly less than the circumference of the stem 31 so that the ends of the assembly will be spaced from one another, as will be noted in l"l(iS. l [03.

The distance d between the leading edge of the collar asscmhly 40 and the electrode 32 (measured along a line parallcl to the stem axis) is critical insofar as it determines the temperature at which the amalgam operates in the completed lamp. This, in turn, determines the mercury vapor pressure within the lamp when the latter is energized and thus controls its light output and operating efficiency.

Any metal which amalgamates readily with mercury and which will no contaminate the lamp atmosphere at the temperatures which will be encountered during the assembly and operation of the lamp can be used. Examples of metals which meet both of these requirements and are suitable include thallium, tin, and alloys thereto. Indium is particularly suitable, as hereinafter disclosed.

It should also be noted that the overlying strips 42 and 43 of cloth can be woven from other materials besides metal wire. it should, however, be fabricated from a material that is substantially inert with respect to both mercury and the amalgamforming metal and should also be one which the amalgam will wet. The cloth can, accordingly, be made from glass or quartz fibers or from nickel, nickel-plated iron, aluminum, titanium, or iron wire. Grade A nickel cloth has given excellent results and is preferred.

Any suitably foraminous material, such as perforated sheets of metal, can also be used in place of the cloth strips.

METHOD OF FABRICATING VAPOR CONTROL ASSEMBLY in FIG. 6 there is shown a preferred method for fabricating the laminated vapor-pressure control assembly 40 described above. As shown, the preferred technique involves positioning a strip 44 of a suitable amalgam-forming metal, such as indium, between two strips 42 and 43 of foraminous material (nickel wire cloth, for, example, as indicated) and feeding the strips while in such overlying relationship between a pair of rollers 46 and 47 which rotate and compress the respective strips one against the other. Since the strip 44 of amalgamforming metal is very ductile and soft, the foraminous strips 42 and 43 are partly embedded in the strip 44. As a result, the overlying strips of foraminous material adhere to and are held together solely by the interposed layer of amalgam-forming metal thus providing a lamination 40 that can be readily handled without falling apart. This method of fabrication is very advantageous since continuous strips of material can be automatically fed into the rollers to produce a continuous lamination that can then be cut into the required lengths.

MOUNT AND LAMP ASSEMBLY After the lead wires, electrodes and anodes have been joined with the glass stem 3| in the usual well-known manner to form the mount 30, he laminated vapor-pressure control assembly 40 is simply wrapped around the stem and clamped in the proper position thereon by the wire ring 41, as described previously and shown FIGS. 2 and 3. If desired, the assembly and ring can be combined to form a unit that can be slipped onto the stem. The mount 30 is then sealed into one end of the envelope 29 in the regular manner. When this operation is completed, the tubulated mount is sealed into the other end of the envelope which is then evacuated, charged with a suitable fill gas, dosed with a predetermined quantity of mercury and tipped off in accordance with standard lampmaking The mercury combines with the metal strip 44 in the tipped-off lamp to form an amalgam of the desired composition.

Since the vapor control assembly 40 is fastened to one of the mounts 30 before it is sealed into the envelope 29 and at this stage contains only the amalgam-forming metal 44, the lamp 28 can be manufactured in the conventional manner and no special precautions are necessary to protect the assembly from heat etc. during lamp manufacture as in the prior art lamps when the amalgam was formed prior to lamp assembly. The composition and physical properties of the amalgam that is subsequently formed within the completed lamp are determined by the quantities of mercury and amalgam-forming metal that are separately placed within the lamp as it is being fabricated. The amount of amalgam-forming metal is determined by the dimensions of the strip 44 that is placed in the laminated assembly 40, and the amount of mercury is regulated in the usual manner by the dosing operation.

The vapor-pressure control assembly 40 is preferably placed on the nontubulated mount 30, as shown in FIGS. 1 to 3, to avoid the accidental loss of amalgam-forming metal during lamp manufacture. Experience has shown that this can occur when the lamps are fabricated on a sealex machine of the type in which the tubulated mount is first sealed into the lower end of the envelope 29 while the latter is held in a vertical position, and the envelope is then inverted to seal the nontubulated mount into the other end of the envelope. When the assembly 40 was attached to the tubulated mount, occasionally some of the heat-softened amalgam-forming metal would be torn free from the assembly as the envelope was swung through an arc and being inverted in preparation for the second sealing-in operation. Placing the assembly 40 on the nontubulated mount which is sealed into the envelope after the envelope has been inverted thus avoids this potential manufacturing problem and insures that an amalgam of the desired composition will be formed within the completed lamp.

Another important feature of the preferred embodiment of the invention is the provision of a margin 45 (see FIG. 4) along each side edge of the assembly 40 that is initially free from amalgam-forming metal. As is shown more clearly in FIG. 7, this can readily be accomplished by using a sheet 44 of an amalgam-forming metal, such as indium, that has a width S which is smaller by a predetermined amount than the width W of the strips 42 and 43 of nickel wire cloth or other foraminous material that is used. The metal sheet 44 is approximately centered with respect to the cloth strips to provide margins 45 of a predetermined width along each edge of the assembly 40 that is free from the amalgam-forming metal.

Experience has shown that when the amalgam-fonning metal 44 extends to the edges of the clothe strips 42 and 43, it tends to collect in droplets along the edge of the assembly 40 when the lamp is handled in the factory while still hot. Occasionally, some of these droplets would be jarred loose from the assembly 40 while the lamp was still being processed or tested resulting in a loss of metal from the assembly and improper mercury vapor pressure regulation. The provision of the metal-free margins 45 along both sides of the assembly 40 avoids this problem in that the amalgam-forming metal, even though it wets the cloth strips, takes a considerably time to migrate to the edges of the assembly. The margins, accordingly, provide a sort of buffer zone that retains the amalgam-forming metal on the assembly 40 during the subsequent fabrication and testing operations performed on the lamp. While the amalgam that is formed eventually migrates by capillary action to the edges of the assembly 40, this occurs long after the lamp has been completed and has been in use and is thus no longer subjected to mechanical impacts that would tend to jar the amalgam loose from its support structure.

SPECIFIC EXAMPLES In order to achieve proper control of the mercury vapor pressure within the lamp and be practical from a manufacturing standpoint, the amalgam-retaining support structure of wire cloth or other foraminous material that is used must have the following characteristics:

a. The support structure or assembly must expose a sufficient amount of the amalgam-fanning metal to the mercury vapor within the lamp to establish dynamic equilibrium and optimum output within a short period of time, as for example within 5 to minutes.

b. The strips of foraminous material must provide a matrix having sufficient volume to contain the required amount of amalgam without the formation of teardrops.

c. The openings in the foraminous material must be of a size such that the amalgam, when in a semlliquid and liquid state, will fill the openings by capillary action.

d. The foraminous strips must be fabricated from a material that is substantially inert with respect to both mercury and the amalgam-forming metal employed, and it must also be one which the amalgam will wet.

In addition, it is desirably that the relative dimensions of the foraminous strips and sheet of amalgam-forming metal be such that a margin that is initially devoid of metal will be provided along both edges of the completed assembly when the metal sheet is centrally located therein, as described above.

Experience has shown that all of the aforesaid requirements are met by fabricating the foraminous strips from a cloth woven from Grade A nickel wire approximately 0.009 inches in diameter having 50 meshes per linear inch in each direction. The width of the openings in the cloth are thus approximately 0.0l 1 inch and the open area of the cloth is about 30.3 percent. In the case of indium two strips of nickel cloth [5 inches long and three-eighths of an inch wide, when combined with a strip of indium approximately one-fourth of an inch in width and 0.01 1 inch thick and the same length as the cloth strips, provided an assembly that contained approximately 500 milligrams of indium and a metal-free margin along each edge of the asembly approximately one-sixteenth of an inch wide. When this assembly was mounted on the stem of a 96-inch l,500-milliampere fluorescent lamp having an envelope approximately 1% inches in diameter (T 12) that was dosed with about 120 milligrams of mercury, an amalgam containing approximately percent by weight of indium (approximately 88 atomic percent indium) was formed within the lamp that maintained the mercury vapor pressure within the desired limits.

Since the mercury dosage required for satisfactory lamp life is proportional to the surface area of the bulb, a mercury vapor control assembly constructed as described above but only half as long (three-fourths of an inch) would be suitable for use in a 48-inch T I2 highly loaded lamp of the same current rating containing approximately 60 milligrams of mercu- For a 96inch T 12 lamp of the aforesaid type, the minimum amounts of indium and mercury that can be used consistent with adequate lamp life are 222 milligrams and 100 milligrams, respectively. For 48-inch T 12 lamp, the corresponding values are 50 milligrams of mercury and l I l milligrams of indium. In the case of a 72-inch T 12 lamp at least 166 milligrams of indium and 75 milligrams of mercury would be required.

ALTERNATIVE EMBODIMENTS FIGS. 8 TO 2| In FIGS. 8 and 9 there is shown another form of lamp mount 30a and vapor-pressure control assembly 40a that are identical to those shown in FIGS. 2 and 4, respectively, except that the ends of the laminated assembly are cut at an angle relative to the longitudinal axis of the assembly rather than at right angles thereto. The ends of the assembly 400 are thus tapered in opposite directions as shown in FIG. 9 and, when the assembly is mounted on the stem 310, the end edges are spaced from and extend parallel to one another along a line that is transverse to the stem axis, as shown in FIG. 8. This construction serves to prevent any droplets of amalgam, or amalgam-forming metal, that may form at the cut ends of the assembly from falling free when the lamp is held in a vertical, or tilted upright position, while the lamp is still hot. Since the tapered cut end edges of the assembly lie one above the other throughout substantially their entire length, even when the lamp is held in a vertical position, any droplets that form along and fall from the uppermost edge will strike and be absorbed by the lowermost edge and thus be retained on the assembly.

In FIG. 10 there is shown another form of mount 3% wherein the collarlike vapor control assembly 40b is held in encircling but spaced-apart relationship with the stem 31b by an L-shaped support wire 48 that is fastened to one of the lead wires (at a point adjacent to the stem press) and to the assembly, as by welding to one of the metal cloth strips of the assembly. As shown more particularly in FIG. II, this arrangement physically isolates the amalgam-containing assembly 40b from the stem 31b and minimizes the effect of variations in the ambient and stem temperature on the operating temperature of the amalgam.

If desired, the amalgam-containing assembly 40b may also be electrically isolated from the electrode 32b by utilizing an L-shaped support wire consisting of two segments 49 and 50 that are joined together by a glass insulator 51 as illustrated in FIG. 12. In this case, the assembly 40b would not only be physically spaced from the stem tube but would be disposed in electrically floating relationship with respect to the electrode structure.

The same physical and electrical arrangement can also be obtained by providing a mount 30c having an L-shaped support wire 52 that is anchored in the stem press 370, as shown in FIG. 13.

If desired, heat transfer to the vapor-pressure control assembly 40c may be further reduced by using a support wire 53 having an intermediate retroverted or looped portion 54, as illustrated in FIG. 14. This type of support would provide a longer heat path from the stem to the amalgam-containing assembly and thus serve to further isolate the amalgam from temperature variations induced in the stem by changes in the ambient temperature.

In FIG. I there is illustrated still another arrangement for mounting the pressure control assembly in spaced-apart encircling relationship on the stem. According to this embodiment, the arcuate assembly 40d is held in the desired position by a resilient wire ring 55 that is clamped on the stem 31d and has a pair of laterally extending arms 56 that extend through and are interlocked with the assembly. As shown in FIG. 16, the laminated assembly 40d is provided with a pair of slotlike apertures 57 that are dimensioned and spaced to receive the hooked ends of the arms 56 and then hold them in such a position that the circular portion of the ring 55 is contracted and firmly locked in place around the stem 31d.

In FIGS. I7 to I9 there is illustrated still another embodiment wherein the assembly-holding means includes bimetal elements 59 and 6] that are arranged to shift the position of the vapor control assembly 40c relative to the electrode 32 in response to variations in temperature within the lamp. The holding means in this case comprises a wire support 58 that is attached to the arcuate amalgam-containing assembly 40c and to one end of a generally U-shaped bimetallic element 59, the other end of which is connected by means of an arcuate support wire 60 that curves around the stern 31c (see FIG. 19) to the end of another U-shaped bimetallic element 61. The opposite end of this element is, in turn, secured to the upper part of the mount 30 by another support wire 62 that is embedded in the stern press 372.

The aforesaid bimetallic elements 59 and 61 are adapted and arranged to form an elongated and movable heat-sensitive support member that shifts the assembly 40:! toward and away from the electrode 32c along the stern 3Ie in response to temperature variations within the lamp induced by changes in the ambient temperature. Thus, when the ambient temperature is 120 F. for example, the assembly 40e will be spaced from the electrode 32: by distance 41 (as indicated in FIG. I7) and will be automatically shifted toward the electrode and be spaced therefrom a distance d,,,,,,,,,,,,,,, when the ambient temperature drops to a predetermined value, as for example 70 F. This arrangement, accordingly, automatically compensates for variations in the ambient temperature conditions and permits the operating temperature of the amalgam to be maintained within very close limits.

Another temperature-compensating arrangement is shown in FIG. wherein the arcuate mercury vapor control assembly 40] is held in spaced-apart and encircling relationship with the stem 31 f at a fixed distance from the electrode 32]" by a relatively heavy support wire 64 that is anchored in the press 37] of the mount 30f and fastened, as by welding, to the assembly. An arcuate heat shield 65 is suspended between the assembly and electrode by a generally U-shaped bimetallic element 66 that is secured to the support wire 64, as is shown in the drawing. This bimetallic element is so arranged that at a preselected ambient temperature it exposes the amalgam-containing assembly 40] to the radiated and convected heat emanating from the electrode 32f and the discharge of the energized lamp. This condition is illustrated by the solid-line showing of the bimetallic element 66 and shield 65 in FIG. 20. However, when the ambient temperature increases, the bimetal flexes and shifts the shield into masking relationship with the assembly (as indicated by the dotted line showing of the bimetallic element and shield) thereby reducing the heating affect of the electrode and are stream on the assembly and maintaining the amalgam within the desired temperature range.

In FIG. 21 there is shown still another form of the invention wherein mercury vapor pressure control assembly 403 is sup ported in spaced-apart relationship with the walls of the envelope 29 by a compressible wire clip 68 of generally U- shaped configuration that engages the envelope wall at a plurality of spaced points. The clip, when in relaxed condition, is slightly larger than the inner diameter of the envelope so that it is held in the desired position inside the lamp solely by the compressive force exerted by the envelope. The retaining clip is preferably of such configuration that the amalgam-containing assembly 40g is suspended at a location between the center of the envelope and the envelope walls, as shown in FIG. 2], so as to avoid placing the amalgam in the main portion of the arc stream. If desired, more than one amalgam-containing assembly may be attached to the clip, as indicated by the phantom showing of additional assemblies on the upstanding portions of the clip as viewed in FIG. 2].

In addition to the various holding support structures and arrangements described above, the vapor-pressure control assembly may also be secured directly to the envelope wall at a preselected position along the bulb axis. This may be accomplished by providing a tab extension on the assembly, for example, and securing this to the envelope wall by a suitable cement that will not contaminate the lamp atmosphere.

EFFECTS OF AMALGAM COMPOSITION AND LOCATION OF LAMP PERFORMANCE Comparative lamp tests have shown that the composition of the amalgam is critical with regard to both the mechanical design and the performance of the lamp. In order for the lamp to be commercially practical on both counts, the amalgam must be retained at the desired location within the lamp even though the amalgam may be converted into a liquid at the temperatures that prevail within the lamp when the latter is energized. On the other hand, when the lamp is not energized the amalgam must be sufficiently rigid or stiff to remain on or within the support structure on which it is placed so that the lamp can be handled and shipped without dislodging the amalgam.

In the case of an indium-mercury amalgam, for example, it has been discovered that the amount of indium in the amalgam must be maintained within the range of about to about atomic percent. As shown in the phase diagram for the mercury indium alloy system illustrated in FIG. 22, an amalgam containing this amount of indium remains in either the solid or liquid-solid phase over a temperature range from 4 F. and below to at least about 122 F. Thus, If an amalgam of this composition is placed within the lamp it will remain in a substantially solid state and will not be jarred loose from its support structure while the lamp is being handled during manufacture or shipment. The aforesaid range of temperature is indicated by the lower hatched region 70 in the graph and is identified as the nonoperating range.

An indium-mercury amalgam containing from about 80 to 95 atomic percent indium and operating at temperatures from about 122 F. to 266 F. will tend to liquify. However, even when in a liquid state such an amalgam will be retained on a suitably designed foraminous support structure, such as the type described previously. Thus, the amalgam will be held in the proper location within the lamp while the latter is operated in its fixture. This temperature range is indicated by the upper hatched region 72 on the graph and is identified as the operating range." As will be noted, most of the combinations of indium-mercury amalgam compositions and operating temperatures provide an amalgam that remains in either the solid or liquid-solid state.

While there is a considerable overlap of the aforesaid ranges, as indicated by the cross-hatched region 74 on the graph, it will be appreciated that an amalgam that is sufficiently rich in indium can be retained in a properly designed holding assembly over a range of temperatures broad enough to enable the lamp to be operated under an extremely wide range of ambient temperatures and power loadings.

This is indicated by the graph shown in FIG. 23 wherein the variation in the operating temperature of an amalgam located on the bulb wall and on the stem versus ambient temperature is shown for atypical 96-inch T 12 highly loaded (1,500 ma.) lamp operated bare in still air. The temperature of the amalgam is much higher when it is located on the stem (curve 76) rather than the bulb wall (curve 78). However, since the two curves are substantially parallel, it is apparent the temperature of the amalgam is affected in substantially the same manner by changes in ambient temperature, regardless of the part of the lamp structure on which the amalgam is placed. Thus, at an ambient temperature of 110 F. the operating temperature of the amalgam when located on the stem approximately 40 millimeters from the electrode will be about 230 F. in this particular type of lamp. (as indicated by curve 76), which is well within the operating range referred to above and indicated by the hatched region 72 of FIG. 22. In addition, the operating temperature of the amalgam on the stem can be varied over a considerable range (for example down to about 200 F. at the aforesaid ambient temperature of 110 F.) simply by shifting its position on the stem and increasing or decreasing the distance between the amalgam and the electrode. The degree of temperature control afforded by adjusting the position of the amalgam on the lamp stem is indicated by the shaded region 79 in FIG. 23, which region straddles the curve 76 and spans a range of about 40 F. This temperature difference was obtained by varying the amalgam-electrode spacing over a range of approximately 8 millimeters, that is, from 34 millimeters to 42 millimeters. The operating temperature of the arnalgam can be controlled over a wider range by increasing the amalgam-electrode spacing a corresponding amount, and by using a longer stem if necessary.

As indicated by the dotted line portions of the curve 76 and shaded region 79, the amalgam, when placed on the stem 34 millimeters from the electrode, will not reach a temperature of 266 F. until the ambient temperature is increased to about l40 P. On the basis of this extrapolated portion of the graph, satisfactory control of the mercury vapor pressure can be maintained at ambient temperatures ranging from about 40 F. to l40 F. even when the amalgam-electrode spacing is quite small.

The marked improvement in the percent brightness maintained at various ambient temperatures by using an indiumrich amalgam is indicated in the graph shown in FIG. 24. The curve 80 represents the change in the light output exhibited by a conventional highly loaded lamp having and chambers when the lamp is operated at various ambient temperatures. As will be noted, the conventional lamp had a peak output at an ambient temperature of about 70 F. (still air) and dropped to 90 percent brightness at ambient temperatures of approximately 48 F. and 98 F. The conventional lamp thus maintained 90 percent of its peak output over an ambient temperature range of about 50 F.

In contrast, the corresponding curve 82 of a highly loaded lamp of the same type and dimensions containing an amalgam having approximately 87 atomic percent indium shows that this lamp had a peak output at about 80 F. ambient (still air) and maintained 90 percent of its peak output from about 42 F. to about 130 F. ambient, or over a range of 88 F. The brightness versus ambient temperature curve 82 of the indium-mercury amalgam lamp is thus much flatter and wider than the corresponding curve for the conventional lamp utilizing end chambers and condensed mercury as the vaporpressure control center. In order to maintain the brightness at 90 percent of the peak the mercury vapor pressure must be maintained between about 3 and 14 microns. It will accordingly be apparent that the indium-mercury amalgam maintained this pressure over a wider range of ambient temperatures and markedly improved the lamp performance under varying ambient temperature conditions, as is indicated by the relative sizes of the hatched and cross-hatched areas under the respective curves in FIG. 24.

The use of an indium-mercury amalgam containing at least 80 atomic percent indium aflords an additional advantage in that the brightness of the lamp is less affected by changes in ambient temperature as the atomic percent of indium in the amalgam is increased above this value. This is indicated by the curves 83, 84 and 85 in FIG. 25 which illustrate the manner in which the brightness of identical lamps, designed to peak at the same ambient temperature, varied as the atomic percent of indium in the amalgam was increased from 80 percent to 87 percent and finally to 92.5 percent, respectively. As is indicated, the lamp containing amalgam having 92.5 atomic percent indium was much flatter and maintained 95 percent brightness over a significantly wider range of ambient temperature (about fl5 F.) than any of the other lamps. Thus, from an operational standpoint it is preferred that the atomic percent of indium in the amalgam be made as high as practical.

This broadening elTect of indium-mercury amalgam on the output versus ambient temperature characteristic of the lamp is not only unique but is extremely advantageous in that the amalgam becomes less liquid as the indium content is increased (see FIG. 22) and will thus exhibit less of a tendency to become dislodged from the retaining assembly when the lamp is operated.

The flexibility of lamp design afforded by the present invention is illustrated in FIG. 26 which illustrates how the peak output versus ambient temperature characteristics of a given lamp can be shifted simply by varying the spacing between the amalgam and the electrode. As indicated by the curve 86, with an electrode-amalgam spacing of 34 millimeters the lamp peaked at an ambient temperature of about 75 F. (still air), whereas peak output occurred at about 97 F. ambient with a spacing of 38 millimeters (curve 87) and at about US F. ambient when the spacing was increased to 42 millimeters (curve 88). The lamps on which these curves are based were 48-inch T 12 lamps having a current rating of 1,500 ms. and an indium-mercury amalgam containing 85 atomic percent indium. Thus, by properly correlating the amalgam composition and the arnalgam-electrode spacing, it will be apparent that lamps can be designed to peak at the particular ambient temperature desired within the range of operating temperatures usually encountered in the application of such lamps. As indicated by this family of curves, 90 percent of peak brightness can be obtained at ambient temperatures ranging from about 40 F. to 160 F. by properly adjusting the amalgam-electrode spacing.

The marked improvement in lamp performance obtained by using an indium-rich amalgam in accordance with this invention is also illustrated by the graph shown in FIG. 27. in the graph, the ambient temperature range within which percent of peak brightness is maintained is plotted against the atomic percent indium present in the amalgam. As indicated by point E, a conventional 48-inch T [2 highly loaded fluorescent lamp having end chambers and pure mercury (0 atomic percent In) maintained 95 percent of its peak brightness over an ambient temperature range of 25 F. The corresponding value for a conventional 96-inch T 12 lamp having end chambers is indicated by point 1 and is approximately 33'' F. A 96-inch T 17 Power Groove lamp employing a cooled region and condensed mercury as the vapor-pressure control center maintained 95 percent of its peak output over ambient temperature range of 3 P F. (point K on the graph).

In contrast, highly loaded 96-inch T 12 lamps containing an ln-Hg amalgam with 80 atomic percent, 87 atomic percent and 92.5 atomic percent indium sustained 95 percent of peak output over an ambient temperature range of 42 F., 55 F., and 67 F., respectively, (points In, lb, and lc). As indicated by the upward slope of the curve 90 drawn through these points, the brightness is maintained at 95 percent of the peak over progressively wider ranges of ambient temperature as the amount of indium in the amalgam is increased. Hence, a highly loaded lamp capable of maintaining its output over a much wider range of ambient temperatures than was heretofore posible with a condensed-mercury control center can be designed by properly adjusting the indium content of the amalgam.

it will be appreciated from the foregoing that the objects of the invention have been achieved insofar as a novel and very convenient means for controlling the mercury vapor pressure within a highly loaded fluorescent lamp or other gaseous discharge device has been provided. The amalgam-retaining structures are such that they can be readily fabricated and assembled with the lamp components without interfering in any way with the regular sequence of lamp-making operations.

While a number of lamp embodiments and a preferred method of fabricating an amalgam-retaining assembly have been described, it is to be understood that various modifications can be made without departing from the spirit and scope of the invention.

I claim as my invention:

l. in combination with a low-pressure mercury vapor discharge lamp that initially contains a predetermined amount of mercury, means for controlling the mercury vapor pressure within said lamp during the operation thereof comprising a quantity of indium that is disposed at a predetermined location within said lamp such that said indium combines widr substantially all of the mercury when the lamp is deenergized and forms an amalgam which initially contains between 88 and about 95 atomic percent indium and regulates the mercury vapor pressure within the lamp during operation thereof, and means holding the formed amalgam at said predetermined location within the lamp.

2. A highly loaded fluorescent lamp that is approximately 48 inches long and 1% inches in diameter and initially contains at least 50 milligrams of mercury and at least 11] milligrams of indium which are combined with one another and constitute an amalgam, the quantities of indium and mercury being so related that said amalgam initially contains between 88 and about 95 atomic percent indium, said lamp having means secured therein which holds the amalgam at a predetermined location within the lamp such that said amalgam controls the mercury vapor pressure during lamp operation.

3. A highly loaded fluorescent lamp that is approximately 72 inches long and 1% inches in diameter and initially contains at least 75 milligrams of mercury and at least 166 milligrams of indium which are combined with one another and constitute an amalgam, the quantities of indium and mercury being so related that said amalgam initially contains between 88 and about 95 atomic percent indium, said lamp having means secured therein which holds the amalgam at a predetermined location within the lamp such that said amalgam controls the mercury vapor pressure during lamp operation.

4. A highly loaded fluorescent lamp that is approximately 96 inches long and I it lnches In diameter and initially contains at least I milligrams of mercury and at least 222 milligrams of indium which are combined with one another and constitute an amalgam, the quantities of indium and mercury being so related that said amalgam initially contains between 88 and about 95 atomic percent indium, said lamp having means secured therein which holds the amalgam at a predetermined location within the lamp such that said amalgam controls the mercury vapor pressure during lamp operation.

5. A low-pressure mercury vapor discharge lamp adapted for operation at a loading such that the mercury vapor pressure would normally exceed that required for optimum efficiency comprising:

an elongated light-transmitting envelope containing an inert ionizable fill gas,

an electrode sealed into each end of said envelope,

a support structure mounted within said envelope at a predetermined location therein spaced from said electrodes and the discharge path therebetween, and

an amalgam of a metal with mercury that wets and is anchored to said support structure, said amalgam by virtue of its affinity for said support structure and the location thereof relative to said electrodes and discharge path having an operating temperature such that it remains on said support structure and controllably lowers the mercury vapor pressure within said lamp during the operation thereof.

6. A low-pressure mercury vapor discharge lamp as set forth in claim 5 wherein said support structure comprises a resilient clip that engages the inner surface of said envelope at spaced points and is retained within said envelope at a predetermined location along its axis solely by the compressive force exerted on the clip by said envelope.

7. A fluorescent lamp comprising:

a tubular light-transmitting envelope,

a vitreous stern sealed to and extending inwardly from each end of said envelope,

a pair of lead wires sealed through each of said stems and extending therefrom into said envelope,

an electrode attached to the inwardly projecting end segments of each pair of lead wires,

a foraminous support structure held by one of said stems at a predetermined location between the respective electrode and end of said envelope, and

an amalgam of a metal with mercury that wets and is carried by said support structure, said amalgam extending over a considerable portion of said support structure and at least partly filling the openings thereof.

8. A fluorescent lamp as set forth in claim 7 wherein said support structure comprises a piece of metal wire cloth.

9. A fluorescent lamp as set forth in claim 7 wherein said support structure is clamped on said stem in encircling relationship therewith.

10. in combination with a fluorescent lamp having an elongated envelope:

a mount structure including a stem sealed into one end of said envelope,

a pair of lead wires sealed through a press on the inner end of said stem,

an electrode fastened to the inwardly projecting end segments of said lead wires,

and arcuate vaporacontrol assembly comprising a layer of an amalgam of a metal with mercury disposed between two strips of wire cloth, and

means holding said arcuate assembly in encircling relationship with said stem at a location such that said assembly is spaced a predetermined distance from the transverse plane that passes through the portions of said lead wire segments that are fastened to said electrode.

II. The combination set forth in claim I0 wherein said holding means comprises a resilient annular member that overlies said arcuate assembly and firmly clamps it around said stern.

l2. The combination set forth in claim 10 wherein the length of said arcuate assembly is slightly less than the circumference of said stem so that the ends of the assembly are thus proximate to but spaced form one another, and the said proximate to but spaced from one another, and the said proximate ends of the assembly are tapered in opposite directions and disposed in generally parallel relationship with a line that extends transverse to the stern axis.

13. The combination set forth in claim 10 wherein said holding means comprises a support wire that is fastened to one of said lead wires and to said assembly and holds the latter in encircling but spaced-apart relationship with the stem.

14. The combination set forth in claim 13 wherein said support wire consists of two wire segments that are joined together by an insulator which electrically isolates the vaporcontrol assembly from the lead wire to which it is attached.

15. The combination set forth in claim wherein said holding means comprises a support wire that is anchored in the stem press and is fastened to and holds said assembly in encircling but spaced-apart relationship with the stem.

16. The combination set forth in claim wherein said holding means comprises a support wire having one end anchored in said stem press and its opposite end fastened to said assembly, and said support wire has a retroverted intermediate portion of such configuration that said assembly is disposed in encircling but spaced-apart relationship with said stem.

17. The combination set forth in claim 10 wherein said holding means comprises a resilient annular member that is clamped in encircling relationship on said stem and has a laterally extending leg that is attached to said arcuate assembly and supports it in encircling but spaced-apart relationship with said stem.

18. The combination set forth in claim 10 wherein said holding means comprises a plurality of wires and a pair of bimetallic elements that are joined together and constitute an elongated heat-sensitive support member having one end secured to said mount structure and its opposite end fastened to said assembly, said support member having a configuration such that said assembly is disposed in encircling but spacedapart relationship with said stem, and said bimetallic elements being arranged and operable in response to temperature variations within said lamp to move the vapor-control assembly along said stern toward and away from the electrode and thereby to minimize the efi'ect of such temperature variations on the amalgam carried by said assembly.

19. The combination set forth in claim 10 wherein said holding means comprises (a) a support wire having one end fastened to said mount and its opposite end fastened to said as sembly, and (b) a shield member that is suspended between said assembly and the electrode by a bimetallic element that is fastened to said support wire, said bimetallic element being arranged and operable in response to variations in temperature within said lamp to move the shield member into and out of masking relationship with said assembly.

20. An electric discharge device comprising, in combination:

a sealed envelope containing an ionizable medium that includes a quantity of mercury,

means for energizing said ionirable medium to arc-sustaining condition,

a forarninous assembly secured at a predetermined location within said envelope, and

a solid body of mercury-amalgamative material disposed within said foraminous assembly,

the foraminations of said assembly being of size to pas mercury vapor and to retain a substantial quantity of said amalgamative material in its fluid state during operation of the discharge device,

said amalgamative material being eflective to control mercury vapor pressure during the operation of the discharge device.

t 8 t i t 

2. A highly loaded fluorescent lamp that is approximately 48 inches long and 1 1/2 inches in diameter and initially contains at least 50 milligrams of mercury and at least 111 milligrams of indium which are combined with one another and constitute an amalgam, the quantities of indium and mercury being so related that said amalgam initially contains between 88 and about 95 atomic percent indium, said lamp having means secured therein which holds the amalgam at a predetermined location within the lamp such that said amalgam controls the mercury vapor pressure during lamp operation.
 3. A highly loaded fluorescent lamp that is approximately 72 inches long and 1 1/2 inches in diameter and initially contains at least 75 milligrams of mercury and at least 166 milligrams of indium which are combined with one another and constitute an amalgam, the quantities of indium and mercury being so related that said amalgam initially contains between 88 and about 95 atomic percent indium, said lamp having means secured therein which holds the amalgam at a predetermined location within the lamp such that said amalgam controls the mercury vapor pressure during lamp operation.
 4. A highly loaded fluorescent lamp that is approximately 96 inches long and 1 1/2 inches in diameter and initially contains at least 100 milligrams of mercury and at least 222 milligrams of indium which are combined with one another and constitute an amalgam, the quantities of indium and mercury being so related that said amalgam initially contains between 88 and about 95 atomic percent indium, said lamp having means secured therein which holds the amalgam at a predetermined location within the lamp such that said amalgam controls the mercury vapor pressure during lamp operation.
 5. A loW-pressure mercury vapor discharge lamp adapted for operation at a loading such that the mercury vapor pressure would normally exceed that required for optimum efficiency comprising: an elongated light-transmitting envelope containing an inert ionizable fill gas, an electrode sealed into each end of said envelope, a support structure mounted within said envelope at a predetermined location therein spaced from said electrodes and the discharge path therebetween, and an amalgam of a metal with mercury that wets and is anchored to said support structure, said amalgam by virtue of its affinity for said support structure and the location thereof relative to said electrodes and discharge path having an operating temperature such that it remains on said support structure and controllably lowers the mercury vapor pressure within said lamp during the operation thereof.
 6. A low-pressure mercury vapor discharge lamp as set forth in claim 5 wherein said support structure comprises a resilient clip that engages the inner surface of said envelope at spaced points and is retained within said envelope at a predetermined location along its axis solely by the compressive force exerted on the clip by said envelope.
 7. A fluorescent lamp comprising: a tubular light-transmitting envelope, a vitreous stem sealed to and extending inwardly from each end of said envelope, a pair of lead wires sealed through each of said stems and extending therefrom into said envelope, an electrode attached to the inwardly projecting end segments of each pair of lead wires, a foraminous support structure held by one of said stems at a predetermined location between the respective electrode and end of said envelope, and an amalgam of a metal with mercury that wets and is carried by said support structure, said amalgam extending over a considerable portion of said support structure and at least partly filling the openings thereof.
 8. A fluorescent lamp as set forth in claim 7 wherein said support structure comprises a piece of metal wire cloth.
 9. A fluorescent lamp as set forth in claim 7 wherein said support structure is clamped on said stem in encircling relationship therewith.
 10. In combination with a fluorescent lamp having an elongated envelope: a mount structure including a stem sealed into one end of said envelope, a pair of lead wires sealed through a press on the inner end of said stem, an electrode fastened to the inwardly projecting end segments of said lead wires, and arcuate vapor-control assembly comprising a layer of an amalgam of a metal with mercury disposed between two strips of wire cloth, and means holding said arcuate assembly in encircling relationship with said stem at a location such that said assembly is spaced a predetermined distance from the transverse plane that passes through the portions of said lead wire segments that are fastened to said electrode.
 11. The combination set forth in claim 10 wherein said holding means comprises a resilient annular member that overlies said arcuate assembly and firmly clamps it around said stem.
 12. The combination set forth in claim 10 wherein the length of said arcuate assembly is slightly less than the circumference of said stem so that the ends of the assembly are thus proximate to but spaced form one another, and the said proximate to but spaced from one another, and the said proximate ends of the assembly are tapered in opposite directions and disposed in generally parallel relationship with a line that extends transverse to the stem axis.
 13. The combination set forth in claim 10 wherein said holding means comprises a support wire that is fastened to one of said lead wires and to said assembly and holds the latter in encircling but spaced-apart relationship with the stem.
 14. The combination set forth in claim 13 wherein said support wire consists of two wire segments that are joined together by an insulator which electrically isolates the vapor-control assembly from the lead wire to which it is attached.
 15. The combination set forth in claim 10 wherein said holding means comprises a support wire that is anchored in the stem press and is fastened to and holds said assembly in encircling but spaced-apart relationship with the stem.
 16. The combination set forth in claim 10 wherein said holding means comprises a support wire having one end anchored in said stem press and its opposite end fastened to said assembly, and said support wire has a retroverted intermediate portion of such configuration that said assembly is disposed in encircling but spaced-apart relationship with said stem.
 17. The combination set forth in claim 10 wherein said holding means comprises a resilient annular member that is clamped in encircling relationship on said stem and has a laterally extending leg that is attached to said arcuate assembly and supports it in encircling but spaced-apart relationship with said stem.
 18. The combination set forth in claim 10 wherein said holding means comprises a plurality of wires and a pair of bimetallic elements that are joined together and constitute an elongated heat-sensitive support member having one end secured to said mount structure and its opposite end fastened to said assembly, said support member having a configuration such that said assembly is disposed in encircling but spaced-apart relationship with said stem, and said bimetallic elements being arranged and operable in response to temperature variations within said lamp to move the vapor-control assembly along said stem toward and away from the electrode and thereby to minimize the effect of such temperature variations on the amalgam carried by said assembly.
 19. The combination set forth in claim 10 wherein said holding means comprises (a) a support wire having one end fastened to said mount and its opposite end fastened to said assembly, and (b) a shield member that is suspended between said assembly and the electrode by a bimetallic element that is fastened to said support wire, said bimetallic element being arranged and operable in response to variations in temperature within said lamp to move the shield member into and out of masking relationship with said assembly.
 20. An electric discharge device comprising, in combination: a sealed envelope containing an ionizable medium that includes a quantity of mercury, means for energizing said ionizable medium to arc-sustaining condition, a foraminous assembly secured at a predetermined location within said envelope, and a solid body of mercury-amalgamative material disposed within said foraminous assembly, the foraminations of said assembly being of size to pass mercury vapor and to retain a substantial quantity of said amalgamative material in its fluid state during operation of the discharge device, said amalgamative material being effective to control mercury vapor pressure during the operation of the discharge device. 